1. Field of the Invention
The present invention relates to an image forming apparatus, a method of adjusting an image positional deviation, and a computer program product.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile machine, a copying machine, and a multifunction peripheral having functions of these machines, conventionally, for example, there is a liquid-ejection type image forming apparatus that ejects liquid droplets such as ink to a recording medium to record an image. The liquid-ejection type image forming apparatus can use various materials, for example, paper, thread, fiber, fabric, leather, metal, plastic, glass, wood, and ceramic as the recording medium. Herein, it is simply referred to as “recording medium” for easy understanding. As a liquid used for recording an image, various liquids such as ink can be used. In the following explanations, it is referred to as “liquid” or “liquid droplet” to clarify the explanations.
Such a liquid-ejection type image forming apparatus generally moves a sheet intermittently in a sub-scanning direction (a conveying direction), and reciprocates a carriage mounted with a recording head that ejects liquid droplets in a main-scanning direction, thereby recording an image on the sheet.
The liquid-ejection type image forming apparatus forms an image by ejecting the liquid droplets in both outward and homeward directions of the carriage to improve the image forming speed.
However, in the liquid-ejection type image forming apparatus, when an image is formed in both the outward and homeward directions of the carriage, a deviation of landing positions of the liquid droplets ejected from the recording head mounted on the carriage to the recording medium occurs in the outward and homeward directions, thereby causing a problem such that image deterioration occurs. Particularly, when image formation is for ruled lines or the like, deterioration of image quality due to the positional deviation is conspicuous.
Therefore, conventionally, in an image forming apparatus that includes a recording head that ejects liquid droplets and forms an image on a recording medium being conveyed, there has been proposed an image forming apparatus provided with a pattern forming unit that forms a reference pattern formed by a plurality of independent liquid droplets and a pattern to be measured, which is formed by a plurality of independent liquid droplets ejected under an ejection condition different from that of the reference pattern, on a water repellent member having water repellency next to each other in a scanning direction of the recording head, a reading unit including a light-emitting unit that irradiates light to the respective patterns and a light-receiving unit that receives regularly reflected light from the respective patterns, and a correction unit that measures a distance between the respective patterns based on a read result of the reading unit to correct a liquid-droplet ejection timing of the recording head based on a measurement result thereof (see Japanese Patent Application Laid-open No. 2008-229915).
The image forming apparatus that performs such correction of the liquid-droplet ejection timing conventionally measures a time after a pattern reading sensor reads the reference pattern until the pattern reading sensor reads the pattern to be measured, to detect a positional deviation of landing positions by a difference between the measured time and a normal time. Accordingly, a positional deviation adjustment is performed with higher accuracy than positional detection by a main-scanning encoder.
However, in the case of this calculation method, because the actual read rate is not constant due to sine fluctuations of a high frequency occurring in the carriage speed mounted with the reading sensor, a calculated value of positional deviation includes an error in the read rate.
Conventionally, therefore, a pattern in which the reference pattern and the pattern to be measured are arranged so that the pitch thereof becomes an integral multiple of a carriage fluctuation cycle is adopted in order to cancel a carriage-speed fluctuation component.
However, in the conventional technique described in Japanese Patent Application Laid-open No. 2008-229915, the reference pattern and the pattern to be measured, which is formed by liquid droplets ejected under an ejection condition different from that of the reference pattern, are used as the pattern. However, such a state may occur that a positional deviation adjustment value for aligning the positions of the pattern to be measured and the reference pattern is not correct. In this case, the read rates (phases of the read rate) of the reference pattern and the pattern to be read do not match with each other. Therefore, an error due to the rate fluctuations is not balanced out, thereby causing an error from a logical read-rate calculation value by an amount of phase shifting. As a result, the positional deviation may not be adjusted accurately.
Therefore, there is a need for an image forming apparatus and a method of adjusting a positional deviation that are capable of improving accuracy of positional deviation adjustment even if a positional deviation adjustment value is not correct.